1. Field of the Invention
The present invention relates to a contact type image sensor capable of reading an original without contracting it and for use in a small size facsimile equipment or the like.
2. Description of the Prior Art
Since a contact type image sensor is capable of reading an original without using a contracting optical system, it has a shorter optical path and it can be miniaturized. Therefore, it is nowadays widely used in such reading apparatuses as small size facsimile equipment and bar code readers.
While there are various types of contact type image sensors in use, the below described contact type image sensor is arranged such that electric charges stored on its plurality of photodiodes by photoelectric transfer are sequentially read out by means of its blocking diodes functioning as switching elements. Since the photoelectric transfer element and the switching element constituting each sensor element of such a contact type image sensor are formed of diodes of the same structure, it is possible to achieve high packaging density of the sensor elements and simplification of the manufacturing process.
However, for a contact type one-dimensional image sensor to read an original, it is required that the sensor is as long as the size of the original. For example, supposing sensor elements are used at the density of 8 elements/mm, virtually close to 2000 elements are required for reading an original in A-4 size. To read pieces of information stored on these sensor elements, matrix driving is frequently used. Matrix driving is frequently used also a contact type two-dimensional image sensor for reading pieces of information stored on a still larger number of sensor elements used therein.
In arranging a matrix drive system in the contact type image sensor, however, there are formed crossover portions (non-contact portions) where a plurality of electrode circuits and another plurality of electrode circuits make two-level crossings. Since there is interposed an insulator of silicon oxide or the like between one electrode and the other electrode at the crossover portion, the crossover portion functions as a capacitor and, thus, each of the crossover portions introduces stray capacitance. Further, since a great number of the crossover portions are formed in the contact type image sensor with a matrix-drive system, there has been a problem that the response of the sensor elements becomes slow or crosstalks are produced between diodes due to the capacitance existing at the crossover portions.